1. Field of the Invention
This invention relates to the compaction of data encoded into bar code symbols, and in particular to the use of color processing to combine a plurality of bar code symbols into an aggregate bar code symbol and to separate and decode the constituent bar code symbols subsequent to scanning the aggregate symbol via color imaging techniques.
2. Description of Related Art
Bar codes have become broadly accepted as a means for automatically identifying objects. A bar code symbol is a pattern of parallel bars and spaces of various widths that represent data elements or characters. The bars represent strings of binary ones and the spaces represent strings of binary zeros. Generally, the bars and spaces can be no smaller than a specified minimum width which is called a "module" or "unit." The bars and spaces are multiples of this module size or minimum width.
The conventional bar code symbol is "one-dimensional" in that the bars and spaces extend only in a single direction. There has been an increasing need, however, for machine-readable symbols that contain more information than conventional bar code symbols. One approach for increasing the information in machine-readable symbols is to reduce the height of the bar codes and stack the bar codes one on top of each other to create a "stacked" or "two-dimensional" bar code. One such two-dimensional bar code is PDF417, which was developed by Symbol Technologies, Inc. The PDF417 symbology utilizes a variable number of codewords which are discrete representations of data. A complete description of the PDF417 code is contained in U.S. Pat. No. 5,304,786, which is assigned to the same assignee as the present invention and which is incorporated by reference herein. Other two dimensional bar code symbologies include Code 1 and Maxicode, which are referred to as matrix codes.
Devices for scanning bar codes are well known in the art, and typically fall into one of two categories; laser and solid state imagers such as charge-coupled devices (CCDs). Laser scanners comprise a visible laser diode for emitting a laser beam, a scanning component such as an oscillating mirror for sweeping the laser beam in a horizontal and/or raster pattern across the bar code, and collection optics including a photosensor for sensing the light reflected off the target bar code and converting the light energy into an analog electrical signal, the amplitude of which corresponds to the reflectivity of the target bar code. The analog signal is processed, digitized and decoded into data representative of that which had been encoded into the target bar code.
CCD based bar code readers are either one-dimensional or two-dimensional. One-dimensional CCD bar code scanners use a linear array of photosensors to capture an image of a cross section of the entire linear bar code at once and produce an analog waveform whose amplitude is representative of the darkness and lightness of the bars and spaces of the captured image. The electric charge stored in each element of the CCD array as a function of the amount of light sensed by an area covered by each element is shifted out serially to form electric signals for further processing, digitizing and decoding. Two dimensional CCD bar code readers operate similarly to capture an image of an entire two-dimensional bar code symbol at once and process it accordingly. Advantageously, image processing techniques allow such a CCD array to be used to read misoriented bar code symbols. For example, U.S. Pat. No. 5,319,181, issued to the assignee of the present invention, describes a technique to implement a CCD camera to capture a two-dimensional PDF417 symbol, store the image data in memory, and perform virtual scanning of the image data to determine the proper orientation of the symbol and enable successful decoding. The waveform is digitized and decoded in a means similar to laser scanners.
Two-dimensional CCD arrays may also be used to capture the image of a linear bar code for processing, digitizing and decoding. In addition, linear CCD arrays may be used to capture the image of a two-dimensional bar code symbol on a row-by-row basis, where the linear array is mechanically or optically scanned down across the rows of the two-dimensional symbol.
Due to the binary nature of bar codes (i.e. the representation of logical 1's and 0's), both linear bar codes and two-dimensional bar code symbols are printed in two highly contrasting colors, which optimally are black bars printed on a white substrate such as a sheet of paper. The white spaces between the black bars provides a high contrast so that the bar/space edges can be readily detected by the scanner, and digitized and decoded in accordance with the particular symbology implemented. In some cases, product packaging dictates that different color pairs be used; e.g. on some beverage cans, gold and/or red may be used. In all cases, however, two contrasting colors are used to designate the two possible states of information being conveyed by the units of the bar code.
It is a continuous goal to be able to increase the information density of bar code symbols in order to store and convey more information in a given area. In particular, it is desired to use the varying optical properties of colors other than black and white in order provide increased information density in bar code printing and reading.
Multi-colored bar code systems have been proposed in the past. For example, U.S. Pat. No. 3,637,993 discloses a transition code recognition system which uses a three color bar code in which transitions from a first color to a second color, from the second color to a third color, and from the third color to the first color manifest a first binary value, and in which transitions from the third color to the second color, from the second color to the first color, and from the first color to the third color manifest a second binary value. This bar code symbology is therefore transition or edge defined, and does not take advantage of other colors in the spectrum. While this prior art system might be extended to include other colors, such an extension would require the definition of an entirely new symbology and would have to supplant existing symbologies which are popular and in extensive commercial use.
It is therefore an object of the present invention to provide a bar code system with increased information density over existing systems.
It is a further object of the present invention to use multiple colors in a bar code system in order to obtain such increased information density.
It is a further object of the present invention to provide such a bar code system which uses multiple colors to increase information density without requiring a new symbology and which can be used in conjunction with any desired symbology.
It is an even further object of the present invention to provide such a bar code system which uses multiple colors as a means for compacting multiple bar code symbols into an aggregate bar code symbol with increased information density and which has a reading means for separating the aggregate bar code symbol back into its constituent bar code symbols for subsequent decoding thereof.
It is a still further object of the present invention to provide such a bar code system which uses multiple colors as a means for compacting a single bar code symbol into a smaller area and which has a reading means for unpacking the compacted single bar code symbol back into its original format for subsequent decoding thereof.